U.S. Pat. No. 4,090,361 (Terry et al.) discloses the use of a heat cycle in a hydride-dehydride-hydrogen cycle (HDH). The HDH cycle is used as a absorption cycle to provide a very low temperature heat sink for a primary power cycle. The heat cycle involves the heating of the hydrogen leaving the hydride reactor bank upon dehydrating so as to impart a higher energy level prior to charging hydrogen to an expansion device for producing work, e.g., turbine.
U.S. Pat. No. 4,503,682, the content of which is expressly incorporated herein by reference as if specifically recited, discloses a combined cycle low temperature engine system. The combined cycle consists of an absorption refrigeration sub-system in combined cycle relationship with organic Rankine turbine systems. The refrigeration sub-system provides a sub-ambient condenser temperature for a turbine cycle, greatly extending the temperature gradient across which the turbine cycle expands, while much of the heat energy rejected from the absorption refrigeration sub-system is internally recovered within the combined cycle system boundaries by regenerative heat transfer to the circulating turbine medium. The internal regenerative heat transfer reduces the net energy consumed to operate the refrigeration sub-system to the point of being less than the power output increase effected for the turbine sub-system.
Extensive computer simulation studies have indicated that the low temperature engine system concept offers a potential double digit increase in power plant turbine cycle efficiencies, as compared with conventional low pressure steam turbine cycles, when the low temperature engine system is employed in an application where it becomes a bottoming cycle replacement for the low-pressure steam turbine in a conventional "all steam" turbine cycle turbine system. It has also been shown to be capable of increasing the power output yield from geothermal resources whose surface plant operating parameters are in a similar thermal regimen to that of a low pressure steam turbine cycle.
U.S. Pat. No. 5,555,731, which is an improvement of U.S. Pat. No. 4,503,682, also expressly incorporated herein by way of reference as if specifically disclosed, provides a power turbine system which employs turbine injectors to supply additional liquid phase turbine medium to the turbine at the elevated temperatures acquired after that liquid medium has performed its function in the low-temperature engine system of absorbing waste heat from the absorption refrigeration subsystem of the low-temperature engine system.
The present invention is the result of a previously unrecognized capability of being able to improve the power output of the low-temperature turbine sub-system that becomes uniquely available to the turbine cycle when expansion of the thermodynamic medium circulating through the turbine enters the sub-ambient temperature range across which its expansion occurs. A well-known characteristics of all heat engine cycles is the fact that the potential output power they can deliver is related to the amount of heat energy that can be input to the expansion process. The higher the temperature at which it is supplied the greater the power output will become. Uniquely, when the turbine cycle of the sub-ambient turbine in this combined cycle enters the sub-ambient portion of its expansion, there are a variety of external heat energy sources, available at temperatures higher than those occurring in the turbine cycle, from which additional heat energy can be supplied to the expanding medium transiting its thermal range,--even including the cooling water temperature in use elsewhere in the turbine plant.
Use of a "reheat cycle" in steam turbine has been practiced for some time. As steam expands to very low pressures, its isentropic path through the turbine converges toward the saturation curve for steam. In order to take maximum advantage of the thermal gradient available at the site of an installation between the best available external site cooling to condense the expanded vapor at the turbine exit, the exit pressure of the steam must enter a high vacuum condition, commonly in the vicinity of 1.5" Hg.abs (3.81 cm.Hg abs.). Generally, as steam approaches the vacuum level, it has already crossed the saturation curve and is in the process of becoming wet,--i.e.--it is in a mixed phase condition with a moisture content approaching a lower limit of 85% quality. Beyond that limit, the moisture content has an adverse impact effect on the turbine blading and increasingly causes a reduction in output power. To overcome the problem, it has been common practice to remove the expanding vapor from the turbine part way through its expansion cycle to send it back to the boiler for a reheat process. When it is returned from the boiler the second time, again at an elevated temperature, it can continue its expansion isentropically from a higher level of superheat, to arrive at its exit pressure at a higher quality level, with a smaller moisture content to adversely affect blading and efficiency.
In the sub-ambient temperature regimen of the turbine in U.S. Pat. No. 4,503,682, at any point below ambient in its cycle, expanding vapor taken from the turbine can be reheated from a variety of heat emitting sources to furnish additional input energy available in its combined cycle environment without resort to the external heat source supplying the system. The original concept of the low-temperature engine system combined cycle is dependent on its capacity to recover heat energy emitted from the associated absorption refrigeration sub-system by internal regenerative heat transfer. Heretofore, that recovery had been limited to recovery of heat emissions from the absorption refrigeration sub-system by use of very cold condensate returning from the condenser of an ORC turbine en route to its boiler as the cooling stream. In effect, it recovered some of the heat ordinarily rejected to ambient cooling water or air temperature as "waste heat" in a conventional "stand alone" absorption refrigeration system.
The present invention recognizes that heat may be recovered by the expanding turbine medium vapor itself, as it traverses its turbine path, before it is ultimately condensed to its liquid phase beyond the discharge point at the bottom of its path through the turbine, when it became useful as a liquid cooling stream en route to its boiler to repeat its cycle.
Furthermore, by the present invention it has been surprisingly found that use of the expanding turbine media itself in a working system designed to operate in accordance with the parameters indicated and employing the sequence of unit operations as diagramed in FIG. 1. will show a minimum of a double digit efficiency improvement when compared with the net power output of a conventional low-pressure steam turbine supplied with the same input steam source as that assumed as the external heat energy source for the alternative combined cycle low temperature engine system referenced. The reheat cycle of the present invention surprisingly offers both an additional mechanism for internal regenerative recovery of heat energy emissions from the absorption refrigeration sub-system otherwise being wasted externally to ambient cooling water, and also a mechanism for increasing the total heat energy input supplied to the expanding vapor circulating through the organic Rankine turbine cycle path in the turbine sub-system.
It is therefore an object of the invention to provide a method of re-heating the turbine medium in a sub-ambient turbine system in combined cycle relationship with an absorption refrigeration system.
It is a further object of the invention to provide a re-heat cycle, in a low temperature engine system combined cycle which is not dependent on the systems' capacity to recover heat energy emitted from the associated absorption refrigeration sub-system by internal regenerative heat transfer.
It is another object of the present invention to provide a heat and energy efficient method for reheating turbine medium in a sub-ambient turbine system by recovering heat from the expanding turbine medium vapor itself.
Further objects of the present invention will become apparent from the following description of the invention and drawings.